Rotary hearth furnaces or calciners for heat processing volatile containing materials therein are not new as shown in U.S. Pat. No. 3,470,068 to Kemmerer and Buschow; U.S. Pat. No. 3,998,703 to John B. Harrell and U.S. Pat. No. 3,859,172 to Charles R. Wilt, Jr.
In all of the aforesaid U.S. patents, there is shown a furnace or calciner comprising a roof, side walls and a hearth mounted for rotation therein. Volatile containing materials are fed into the furnace enclosure and deposited on the rotary hearth and subjected to a heat treatment while on said hearth. Roof and side wall openings are provided for the feed of air and/or a fuel rich gas within the furnace enclosure. Also, side wall mounted burners are provided to supply heat within the enclosure when such is needed. Operation of the furnace or calciner as shown and described in the aforesaid U.S. patents is substantially as follows. A volatile containing material or a combination of a volatile containing and a non-volatile containing material is fed onto the rotating hearth through a roof mounted port or ports and air and/or a fuel rich gas is admitted into the furnace enclosure through roof and/or side walls ports. Rabbles extend downwardly from the roof of the enclosure to a position in close proximity of the floor of the hearth and when the furnace is in operation, the rabbles progressively advance the materials toward a centrally located discharge port provided in the floor of the hearth. The volatiles evolved from the volatile containing materials undergoing treatment on the hearth rise towards the roof of the enclosure where they will mix with the air and/or fuel rich gas and combust therewith in that portion of the enclosure to provide an oxidizing atmosphere in that portion of the enclosure while maintaining a reducing atmosphere about the materials undergoing treatment on the floor of the hearth. Following combustion of the evolved volatiles with the aforesaid oxidants, the waste gases of combustion exit through a centrally located stack and ultimately into the surrounding atmosphere.
The furnaces or calciners constructed and designed to operate in the manner aforesaid have been proven to be very efficient in operation. However, it shoud be pointed out that in such prior known structures, the direction of travel of the volatile containing materials as well as the direction of the oxidant fed into the enclosure along with the waste gases of combustion all travel in the same direction, that is, the oxidant, waste gases of combustion as well as the material undergoing treatment on the hearth travel toward the central portion of the furnace enclosure and such flow of materials can be stated as being co-current.
In order to further improve on the efficiency of operation of a furnace or calciner constructed generally as aforesaid, certain structural changes have been made thereto and these changes and manner of operation of a furnace or calciner incorporating such changes will be set forth with particularity in the ensuing description of the invention.
With the above in mind, it is one of the objects of the invention to provide a means where an oxidant such as air and/or fuel rich gas fed into the furnace enclosure along with the waste gases of combustion are caused to flow in a direction of travel which is opposite or counter-current to the direction of travel of the materials undergoing treatment on the hearth.
Another object of the invention is to provide a plurality of roof mounted manifolds from which air and/or a fuel rich gas is directed into the upper portion of the furnace enclosure through a plurality of concentrically arranged rows of openings provided in the roof of the enclosure.
Another object of the invention is to provide an exhaust manifold about the periphery of the furnace or calciner enclosure with a means therein whereby the air and/or fuel rich gas along with the waste gases of combustion will be caused to travel radially outwardly of the furnace enclosure and directed into a peripherally mounted exhaust manifold and ultimately into the surrounding atmosphere.
Another object of the invention is to provide a larger number of air and/or fuel rich gas inlet ports in the outer periphery of the roof of the enclosure than at the center and intermediate areas of the enclosure.
Another object of the invention is to provide a means whereby air and/or fuel rich gas is admitted into the furnace enclosure in zonal areas, such that the gas atmosphere can be more strongly reducing at the center of the furnace than at the furnace periphery.
Another object of the invention is to provide a means whereby the temperature within the furnace enclosure can be profiled so as to provide for a lower temperature at or near the outer periphery of the enclosure than at the central or intermediate area of the enclosure.